(A) Field of the Invention
This invention relates to a process for the manufacture of 4,4' dihydroxydiphenyl (biphenol, diphenol) from diphenyl and more particularly relates to such process wherein diphenyl is sulfonated followed by fusion of a sulfonate with a metal hydroxide to form 4,4' dihydroxydiphenyl salt which is acidified to form free 4,4' dihydroxydiphenyl.
(B) History of the Prior Art
Biphenol is useful as a reactant in condensation polymerizations and is particularly useful in forming phenolic type resins having good temperature resistance.
Biphenol was formed in the prior art by various unsatisfactory, complex and uneconomical methods. For example, halogenated diphenyl could be hydrolyzed (U.S. Pat. No. 3,413,341) or diphenyl could possibly be directly hydroxylated with peroxide in the presence of a suitable catalyst (U.S. Pat. No. 3,453,332).
One of the most suitable prior art methods for the preparation of biphenol is disclosed in U.S. Pat. No. 2,368,361 wherein diphenyl is sulfonated to form diphenyldisulfonic acid which is then reacted with sodium hydroxide or sodium salt to form a diphenyldisulfonic acid salt followed by fusion of the diphenyldisulfonic acid salt with an alkali metal hyroxide to form the sodium salt of biphenol followed by quenching in water and acidification to form biphenol. In general, low molar ratios of hydroxide to diphenyl disulfonic acid salt in the fusion were not previously believed possible or at least not desirable since in the absence of substantial excesses of alkali metal hydroxides in the fusion or in the absence of undesirable compounds such as benzene sulfonates to increase fusion fluidity, the reaction mass is solid or almost solid at low enough reaction temperatures to prevent product decomposition since the melting temperature of the alkali metal salts of diphenyl disulfonic acid is above the decomposition temperature of the biphenol salt reaction product.
The process disclosed in U.S. Pat. No. 2,368,361 is an example of such an unsatisfactory process since benzene or another aromatic hydrocarbon of relatively low boiling point is required to be sulfonated subsequent to sulfonation of diphenyl. The resulting benzenesulfonic acid or other sulfonic acid of a low boiling aromatic hydrocarbon permits the fusion to be carried out solely in sodium hydroxide which, previously at low molar ratios in the absence of the additional sulfonic acid or sulfonic acid salt, did not yield fusions which were as fluid and efficient as desirable. U.S. Pat. No. 2,368,361 indicates that the presence of benzenesulfonate permits sodium hydroxide to be present in the fusion in a molar ratio of between 3:1 and 4:1 to sulfonic acid groups in the fusion. The patent fails to directly point out that over half of the sulfonic groups are provided by undesirable benzenesulfonate rather than diphenyldisulfonic acid which is the intermediate to biphenol. The method disclosed in U.S. Pat. No. 2,368,361 therefore requires large quantities of sodium hydroxide in the fusion which are not actually utilized in the formation of the sodium salt of 4,4' dihydroxydiphenyl (biphenol sodium salt). In Example 1 of the patent, in the best case, i.e., 65 grams of NaOH and 44 grams of disulfonate (44 out of 100 total sulfonates), there are about 11.4 moles of sodium hydroxide provided for each mole of diphenyldisulfonic acid salt in the fusion. In addition, at the high fusion temperatures of 350.degree. to 370.degree. C., it is believed that at least some decomposition of the sodium biphenol salt product occurs.
After formation, the process set forth in U.S. Pat. No. 2,368,361 still requires steps to separate biphenol from the phenols resulting from fusion of the benzene sulfonates. The process therefore wastes benzene, wastes sodium hydroxide and is exceedingly complex.
It has been suggested that the potassium salt of diphenyldisulfonic acid could be used in fusion with potassium hydroxide which would lend fluidity to the fusion thus permitting lower mole ratios of potassium hydroxide to be used. Unfortunately, potassium salts of sulfonic acids usually are formed by neutralizing the sulfonic acid with costly potassium hydroxide in order to obtain a good yield of the salt. Potassium hydroxide, is generally formed by electrolytic means from the much more readily available and less costly inorganic potassium salts such as potassium chloride.